In a typical CMP (chemical mechanical polishing) process, a semiconductor wafer is placed face-down under high pressure on a polishing pad in the presence of a slurry. The slurry includes abrasives and chemical components. After the wafer is exposed to the slurry, a chemical reaction occurs between the chemical components in the slurry and the materials in the semiconductor wafer. The chemically reacted surface of the semiconductor wafer is then mechanically polished by the abrasives in the slurry.
At the macroscopic level, when fresh slurry is deposited onto the polishing pad, it stays on the pad temporarily and is supplied to the pad/wafer interface by the rotation of the polishing pad. At the microscopic level, the abrasives are supported by asperities in the polishing pad to remove material at the nano-scale on the wafer.
Many defects can be generated by conventional CMP pads. Such defects include dishing, erosion, thinning, and micro-scratches. Such CMP-related defects are well known in the art of semiconductor processing.
Since the polishing process is influenced by the characteristics of the polishing pad, it is desirable to understand the physics associated with the polishing pad to reduce the likelihood of CMP related defects. Compared to the amount of research that has been performed on CMP slurries, very little research has been performed on the design and fabrication of polishing pads. As will be apparent from the discussion below, the present inventors have characterized a conventional polishing pad and have also invented new polishing pads with new features.
A conventional pad may be made of polyurethane. The region near the contact surface of the conventional polishing pad can have a porosity of 30% to 50% (each pore may have a diameter of about 40 μm to 60 μm). Each pore in the polishing pad is separated or defined by wall structures. Such wall structures may also form asperities having widths of about 10 to about 50 μm. In a conventional polishing pad, there are also peaks and valleys that are continuously regenerated by conditioning.
Based on prior research by the present inventors, the side view of a pad can be categorized into three regions. They include the reaction region, the transition region, and the reservoir region (see FIG. 1(a)). The reaction region is mainly composed of wall structures. In the reaction region, the polishing pad and wafer contact abrasives within the slurry. The reservoir region includes pores which provide a region for holding new slurry. The fresh slurry that is supplied to the reaction region can be temporarily held in the reservoir region. It also flows through the transition region to the reaction region. That is, the transition region, which includes pores and walls, is the region where slurry is transported from the reservoir region to the reaction region.
As shown in FIG. 1(a), a conventional polishing pad has a wavy surface profile consisting of peaks and valleys. The contact between the wafer and pad occurs at the crests of the polishing pad in the reaction region. Fresh slurry temporarily collects in the valleys in the reservoir region and is supplied to the reaction region via the movement of the wafer between the peaks and valleys in the transition region during the polishing process. From a three-dimensional analysis, it was determined that the reservoir region is surrounded by the reaction region. Using these structures, the slurry on the bottom of the polishing pad is efficiently guided to the reaction region.
The degradation of a conventional pad is mainly caused by abrasion in the reaction region and plastic deformation. As a result of the wavy profile associated with a conventional polishing pad, the real contact area increases and the real contact pressure drops rapidly during the CMP process, causing the material removal rate (MRR) to decrease dramatically in the absence of a conditioning process. In addition, pad asperities with convex shapes concentrate stress at the areas where the polishing pad contacts the semiconductor wafer being polished, thus increasing the likelihood of dishing and erosion defects.
It would be desirable to provide for an improved polishing pad that addresses the above problems and other problems, individually and collectively.